1. Field of the Invention
This invention relates to the field of acoustoelectronics. More particularly, the invention relates to a surface acoustic wave (SAW) structure capable of low-loss operation over a relatively wide bandwidth.
2. Description of the Prior Art
In the field of electronics, SAW devices are used in a number of well known signal-processing applications such as signal delay, filtering, signal generation, etc. Generally, SAW devices take advantage of the five-orders-of-magnitude reduction in signal propagation velocity achieved by converting a radio frequency (rf) electromagnetic signal into a surface acoustic wave for propagation in a rigid solid.
In a typical SAW application, an rf signal is first converted by an input transducer into an acoustic surface wave that is confined to the surface of a piezoelectric substrate. The acoustic wave is then coupled to an output transducer for conversion to an electrical signal. These transducers usually consist of thin-film metallic electrode structures deposited on the substrate surface. One common transducer structure comprises an array of interdigital electrodes. The width and spacing of the electrodes are factors that help determine the frequency response of the SAW device and its operation as a signal processor.
Conventional SAW interdigital transducers are bidirectional in that the acoustic energy is transmitted in both the forward and backward directions. However, when a conventional SAW transducer with periodic, parallel electrodes incorporates a properly spaced reflective grating (see FIG. 1), the result is a narrowband, unidirectional structure.
One prior art technique for increasing the bandwidth of conventional interdigital SAW structures consists of tapering the electrode width and spacing (see FIG. 2--it is noted that the tapering angles in FIGS. 2, 3 are greatly exaggerated for clarity). The transducer electrodes are slightly rotated at progressively greater angles with respect to the direction of propagation of the acoustic wave. The electrode tapering and its rotation produces a variable electrode spacing parallel to the propagation path. Consequently, the frequency response, which is a function of electrode spacing, will vary along different but parallel paths across the SAW transducer thereby creating a wideband frequency response. This technique is analyzed in detail in the following report: A. P. van den Heuvel, "Properties of Tapered Surface Wave Transducers," ECOM Research and Development Technical Report, Technical Report ECOM-0197-F, pp. 12-14, 18, 19, 22-25, February 1973.